The electrorefining of copper consists in the simultaneous electrolytic dissolution of copper from a relatively impure anode and the plating of relatively pure copper onto a cathode in a copper sulphate, sulphuric acid electrolyte. The copper anode contains a number of impurities that are separated from the copper during the electrorefining process. Typical impurities are precious metals, nickel, lead, iron, selenium, tellurium, arsenic, antimony and bismuth. Most of the impurities form an insoluble slime which falls to the bottom of the cell and is normally treated for by-product metal recovery. However, some of these impurities, notably antimony and bismuth, are soluble in the electrolyte and accumulate as the electrorefining proceeds. If left unchecked, the antimony and bismuth present in the electrolyte will increase to the point where they contaminate the copper cathode. It is therefore necessary, and in fact a general practice in copper electrorefining, to remove antimony and bismuth from the electrolyte.
Iron is another metal usually present in copper electrolyte. It is well known that iron causes two undesirable phenomena during copper electrorefining, namely 1) the Fe.sup.3+ +e.sup.- .fwdarw.Fe.sup.2+ reaction consumes current, thus reducing current efficiency; and 2) the Fe.sup.3+ +1/2Cu.sup.0 .fwdarw.Fe.sup.2+ +1/2Cu.sup.2+ reaction causes corrosion of the copper cathode especially at the surface where the Fe.sup.2+ is more apt to be oxidized. This is particularly a problem for cathodes employing copper suspension loops. Therefore it is desirable to maintain dissolved iron levels to a minimum and to prevent electrolyte oxidation during copper electrorefining.
In view of the criticality of the concentration of the above elements in copper electrolyte solutions, many processes have been proposed for removing antimony and bismuth from copper refining electrolyte.
Biswas et al. in Extractive metallurgy of copper, 1.sup.rst Edition, Pergamon Press, 1976, 310-312 describes the removal of antimony and bismuth from copper electrolyte by passing the solution through a series of electrowinning cells which precipitates the antimony and bismuth in elemental form. Drawbacks associated with this process are: 1) most of the copper in the electrolyte has to be electrowon first before reducing antimony and bismuth; 2) the volume of electrolyte that can be withdrawn for treatment is limited by the amount of soluble copper (oxide) entering the electrolyte from the anodes; 3) arsine formation, a toxic gas, may occur; and 4) the antimony and bismuth residue produced contains large mounts of arsenic, copper and lead which makes treatment of this residue arduous and complex.
Another process currently in commercial use is ion exchange, such as that disclosed by Dreisinger et al. in Final Report CANMET, project No. 0748, Jan. 20, 1992, and Oda et al. in Metallurgical Review of M. M. I. J., Vol. 3, No. 2, November 1986, Symposium Proceedings. The processes consist in passing some copper electrolyte solution through a resin that is specific for antimony and bismuth. Once the resin is loaded, it is stripped, usually with hydrochloric acid, and washed with water. Problems associated with this process are 1) the consumption of large mounts of reagents, mainly hydrochloric acid; 2) the strip solution has to be treated for antimony and bismuth recovery; 3) treatment of hydrochloric acid and water effluent; 4) the resin has a limited useful life, and must therefore be replaced periodically; 5) Fe.sup.3+ tends to adsorb on the resin, which reduces the resin capacity; and 6) the risk of chloride ion contamination of the electrolyte. An improvement of this process has been proposed in U.S. patent application Ser. No. 08/138,024 filed Oct. 19, 1993, wherein the Fe.sup.3+ is reduced to Fe.sup.2+ before passing the solution in the resin column.
Solvent extraction has been found capable of removing antimony and bismuth from mineral acid electrolytes, as disclosed in U.S. Pat. No. 4,061,564 and U.S. Pat. No. 5,039,496. In such process, the electrolyte is mixed with an appropriate sparingly-soluble organic solvent which preferentially extracts antimony and bismuth. The antimony and bismuth need to be stripped from the solvent in order for the solvent to be reused. The drawbacks for this process are similar to those of ion exchange. Effluents need to be treated, the solvent needs to be replenished and there is a lot of solution mixing and manipulation.
Adsorption onto a low-solubility metal-oxide hydrate has also been proposed in 3,696,012 for removing antimony and bismuth. However, the material needs to be regenerated with either nitric, hydrochloric or sulphuric acid, and the effluent treated.
GB 2,005,646 and JP-197-14583 are concerned with the addition of Bi.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 for removing antimony and bismuth from copper electrolyte. Addition of these species to the electrolyte appears to upset the equilibrium between arsenic, bismuth and antimony, which causes them to precipitate below their original concentrations. From 3 to 10 g/L of either product, or both, along with a minimum of 3 g/L of arsenic is needed for the precipitation to occur.
The copper electrolyte may also be treated with barium, strontium or lead carbonate, as proposed in U.S. Pat. No. 4,157,946. Addition of either of these carbonates is found to precipitate bismuth, and to a lesser extent, antimony. This method however, like many of those described above, consumes significant amounts of reagents and requires treatment of important quantities of residues.
Treatment of copper electrolyte with H.sub.2 S after partial decoppering has also been proposed in U.S. Pat. No. 4,404,071. This approach uses H.sub.2 S gas to precipitate arsenic, antimony and bismuth from a copper electrolyte pretreated to reduce the copper concentration to 10-13 g/L. This remaining copper however is also precipitated with H.sub.2 S.
Adsorption of antimony onto activated carbon is the subject of an article of Toyabe et al. in The electrorefining of copper, TMS-AIME, Warrendale, Pa., 1987, 117-128. This process is in use at the Sumitomo Niihama Copper Refinery in Japan, but is much less effective for bismuth than it is for antimony. Further, important amounts of activated carbon are consumed and treated.
In view of the above discussion, it would therefore be highly desirable to develop a process for the removal of antimony and bismuth. This process should be preferably highly selective toward antimony and bismuth, and require minimal amounts of reagents.